A couple weeks ago while on a ten day trip, we started up the propane water heater and found that it sounded different, quieter, and after bout 20 minutes began to get a hot surface odor from the cabinet where it is mounted, no alarms co or co2. Shut it down to wait for a maintenance opportunity. Yesterday I decided to inspect and clean using compressed air figuring that some spider webs were blocking the gas flow. Its a 4 year old unit and was able to blow some rust out, gas ventura looked freeflowing. Nothing significant. So I removed the direction baffle and tried to blow air directly through the hot gas passages and could not get a good air movement. With a flashlight I could see a very large mud dauber nest blocking 2/3 of the exit hot gas passage. Removed the nest and Reinstalled the baffle and cover and the heater worked perfectly. Mud daubers are very commen in my area due to sandy soil. Go figure. Moral of the story, if it has a different sound and a hot surface smell, don't use your propane hot water heater til a full inspection is done
